The core of a continuous inkjet printer is a droplet generator. This component generates a stream of droplets from a body of ink.
The design of a droplet generator has a known theoretical basis, allied to which there are a number of practical limitations.
The mathematics of dividing an ink stream into droplets has been described by Rayleigh. The underlying mechanism of forming the stream into droplets, a process known as modulation, involves creating instability in the ink steam. Factors which influence instability include ink surface tension, ink density, nozzle diameter, and the wavelength of the vibration used to create the instability, along the jet.
It thus follows that different inks may require a different droplet generator.
Two primary methods of modulation are encountered in continuous inkjet printers. In the first, the ink is directly vibrated within a chamber before being discharged through a nozzle. This is known as pressure modulation. In the second, the nozzle is vibrated with respect to a body of ink in contact with the nozzle. This is known as velocity modulation. In reality, a modulation system may include elements of both pressure modulation and velocity modulation.
Historical experience indicates that a typical droplet generator must produce ink droplets whilst operating in a frequency range of 40-130 kHz. It is also well known that there is a practical upper limit for the speed at which the stream of ink droplets impacts the substrate being printed. In essence, there is a well-understood relationship between frequency, nozzle size and print quality.
In the past, droplet generators have employed acoustic energy derived from piezo electric crystals to generate the instability required to produce the droplets. Typically these generators have been designed and constructed as resonant systems to minimise power requirements and energy loss. However, problems invariably arise with mass-produced resonant systems as variations in the tolerances inherent in any manufacturing process, lead to variations in system resonance. As a consequence of the variations in resonance, existing drop generators typically display a lack of consistency in performance is between units. One method of tuning to compensate for this variability is to change a component of the system, such as the nozzle, until the required performance is achieved. This method is inefficient in that it requires the intervention of a skilled technician. For example, we find that tuning by changing nozzles typically involves discarding a number of nozzles for each printer.
Efforts have been made, in the past, to address the problems inherent in resonant systems. European Patent 0 252 593 describes a droplet generator specifically designed to be non-resonant. This is achieved by forming the components of the droplet generator from acoustically soft materials such as poly(phenylene sulphide). Whilst forming a droplet generator from acoustically soft materials may eliminate resonances, experimental work which we have undertaken suggests that modulation (the control of the droplet generation process) is poor with acoustically soft materials. Further, the efficient use of such materials on a mass-production basis would involve significant tooling costs.
A further example of a non-resonant system is described in U.S. Pat. No. 3,972,474. However, during operation of the droplet generator described in this patent, significant acoustic energy is applied to the column of ink within the generator, and thus the design of the device has to take into account the fundamental resonance frequency of the ink column and hence the speed of sound of the ink. This renders the device sensitive to ink type and means that tuning is inevitably required.
It is an object of this invention to provide a droplet generator, particularly a droplet generator for a continuous inkjet printer, which goes at least some way to addressing the problems described above; or which will at least provide a novel and useful alternative.